1. Field of the Invention
The present invention relates generally to an apparatus that can determine the viscosity and water wettability of a spacer fluid and drilling mud mixture at downhole shear rate conditions. For example, the spacer fluid and drilling mud mixture can be tested under a specified pressure, at a specified temperature, and a shear rate may be applied to the mixture that is substantially identical to the shear rate that will be exerted on the mixture due to downhole geometries and conditions. The apparatus may also provide the viscosity of the spacer fluid and drilling mud mixture under a designated pressure, at a specific temperature, and under a specified shear rate. The apparatus may be used to determine the optimal composition and amount of spacer fluid that needs to be added to drilling mud to improve the rheological properties of the mixture to provide adequate water wettability as well as displacement of the drilling mud during the cementing process of a wellbore.
2. Description of the Related Art
Oil and gas wells typically include steel casing or steel tubing, hereinafter steel casing, cemented into the wellbore to provide support to the wellbore and prevent the wellbore from collapsing. The steel casing also allows for a hydraulic seal to be maintained within the wellbore to hydraulically isolate different sections of the well. The cement is important to support the wellbore formation as well as to help protect the steel casing from being damaged.
The first process in creating an oil or gas well is to drill the wellbore or at least a portion of the wellbore. The industry typically uses a drilling mud to aid in the drilling process to remove the drill cuttings from the wellbore. Drilling mud is generally water based mud or oil based. An oil based drilling mud does not provide sufficient water wettability properties, which may result in improper displacement of the drilling mud during the cementing process as well as possibly a decrease in the shear bond strength of the cement.
After the wellbore has been drilled to the desired depth, the drilling apparatus is removed and steel casing is inserted into the wellbore. The insertion of the steel casing displaces the drilling mud both between the steel casing and the wellbore as well as inside the steel casing. Cement is then pumped into the wellbore to cement in the steel casing. The cement within the wellbore needs to have adequate strength to support axial casing loads that may be present throughout the life of the well. It is known that the shear bond strength of the cement decreases if the wellbore is oil wet during the cementing process in comparison to a wellbore that is water wet. Thus, an oil based drilling mud may lower the shear bond strength of the cement if proper steps are not taken to ensure the wellbore is water wet. A spacer fluid may be added to the wellbore to water wet the wellbore. The addition of a spacer fluid may also be necessary to ensure proper displacement of the drilling mud during the cementing process.
Improper displacement of drilling mud within the wellbore can leave a significant amount of drilling mud at the interface between the cement and the wellbore formation. This can lead to poor cement bonding or multiple problems such as hydrocarbon loss due to migration from one zone to another or to the earth's surface, interactions of the corrosive well fluid to the steel casing, channels of undisplaced mud created “defects” in the cement sheath, and not properly supporting the wellbore. A spacer fluid is often pumped down the wellbore before the cement is pumped into the wellbore to properly displace the drilling mud and water wet the formation and external surface of the casing. Although spacer fluids help in the cementing process, one problem is determining the rheological properties of the spacer fluid to ensure its ability to properly water wet the formation and the casing as well as properly displacing the drilling mud. Additionally, it is difficult to determine the optimal formulation of the spacer fluid, the amount of spacer fluid needed, and/or the amount of surfactant in the spacer that needs to be pumped into the wellbore.
A Water Wetting Capability Tester (WWCT) as set forth in API RP 10B-2, First Edition, may be used to determine the water wettability of a fluid or fluid mixture at atmospheric pressure and at temperatures up to 200° F. The WWCT uses a conductivity probe to determine the water wettability of the tested fluid. It is known in the industry that the water wettability of a fluid may be determined by the conductivity of the fluid. As such, the WWCT includes a conductivity probe placed in the fluid cavity of the apparatus. A spacer fluid may be first tested in the WWCT to determine the conductivity index or water wettability of the spacer fluid. The WWCT can then evaluate the mixture of a known amount of a spacer fluid with a known amount of drilling mud. Typically, enough spacer fluid will be added until the mixture has a water wettability that is substantially equal to the water wettability of the spacer fluid alone.
Although the WWCT may be used to determine the water wettability of the combined mixture, it takes multiple tests. Additionally, the WWCT cannot simulate the downhole conditions above 200° F. and at pressures greater than atmospheric pressure, which are typical conditions that the spacer fluid and drilling mud mixture will be under. The WWCT does include a rotating blade to ensure that the mixture (drilling mud and spacer) are mixed properly during testing. However, while the rotational speed of the blade can be varied to quickly homogenize the added fluids and prevent static areas from forming, the shear rate at which the fluids are being exposed may not be determined due to the geometry of the mixing blades and the configuration of the mixing container. Thus, the WWCT cannot simulate the shear rate under which the fluid mixture will be exposed in the casing or in the casing-formation annulus. The WWCT does include a heater, but the heater is limited to temperatures below to 200° Fahrenheit to avoid boiling the fluid mixture. Tests using the WWCT are conducted at ambient pressure not at pressures present in the wellbore. Presently, no commercial laboratory apparatus can measure continuously the combined effects of temperature and pressure on the spacer to water wet an oil-based coated surface and measure the rheological properties of the spacer, oil-based drilling mud, and/or the combined fluids.
The WWCT also does not provide any information as to the viscosity of the combined mixture. The viscosity of the mixture will vary depending upon the viscosity of the base spacer and the drilling fluids and the proportions of spacer and drilling fluids in the fluid mixtures. The viscosity of the combined mixture at a certain mud to spacer ratio can be determined if desired with a separate apparatus. However, this apparatus can only evaluate a single mixture and cannot capture the change in viscosity as the proportion of spacer and drilling fluid changes. Nor can the existing apparatus conduct this type of evaluation under pressurized conditions.
In light of the foregoing, it would be desirable to provide an apparatus that may continuously determine the Theological properties, such as water wettability and viscosity, under simulated wellbore conditions. Further it would be desirable to provide an apparatus that can be used to simultaneously determine the rheological properties of a second fluid and then determine the rheological properties of a mixture of the two fluids during a real time basis under simulated wellbore conditions. It would also be desirable for an apparatus that may be used to determine the affect specific downhole conditions have on the rheological properties of a fluid.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the issues set forth above.